Nacreous pigments comprising metal oxide-coated mica platelets are now in common use to produce pearlescent luster, metallic luster, and multicolor effects approaching iridescence. The pigments were first described in Linton U.S. Pat. Nos. 3,087,828 and 3,087,829. A description of their properties is included in Pigment Handbook, Volume I, Second Edition pp. 829-858, John Wiley and Sons, N.Y., 1988.
The oxide coating is in the form of a thin film deposited on the surfaces of the mica particle. The oxides in most widespread use at present are titanium dioxides and iron oxides. Others are tin oxides, chromium oxides, and zirconium oxides. Mixtures or combinations of the oxides are also used.
The coating have the optical properties of thin films; thus the color reflected by the pigment arises from light interference which is dependent on the thickness of the coating. Thin TiO.sub.2 coatings produce a whitish reflection which appears pearly or silvery. Reflection colors of gold, red, blue, green, etc. are produced by using progressively thicker coatings. Since Fe.sub.2 O.sub.3 has an inherent red color, mica coated with this oxide has both a reflection color and an absorption color, the former from interference, the latter from absorption of light. The iron oxide-coated mica pigments of greatest commercial interest at present contain iron(III) oxide and have reflection colors ranging from yellow to red. The pigments are referred to as "bronze," "copper," "russet," etc.
For extended use in outdoor applications, such as automotive paints, many types of pigments require treatment so that the paint film in which they are used can resist the effects of ultraviolet light and moisture. Conventional pigmentary titanium dioxide has been stabilized with compounds of, for example, silica, aluminum, chromium, zinc, zirconium, and various mixtures. Similarly, the metal oxide-coated mica pigments require stabilization. However, the behavior of an oxide coated on mica differs from that of the free oxide pigment, as discussed in DeLuca et al., U.S. Pat. No. 4,038,099, column 3, lines 19 to 45. Differences also arise because conventional pigmentary titanium dioxide usually has a crystal size of about 0.2 .mu.m, whereas metal oxide-coated mica particles are thin platelets which may have lengths from 2 to 200 .mu.m, depending on the intended application. Experience in stabilizing conventional pigments is therefore not directly applicable to metal oxide-coated mica pigments.
Metal oxide-mica pigments have been successfully stabilized for outdoor use by deposition of a further coating of a chromium compound. Chromic hydroxide was used in Rieger et al., U.S. Pat. No. 4,134,776, and methylacrylatochromic chloride in Jackson, U.S. Pat. No. 3,832,208. The rutile form of titanium dioxide is preferred for exterior grades of TiO.sub.2 -coated mica. The preparation of rutile-coated mica is described in U.S. Pat. No. 4,038,099.
An inherent disadvantage of the chromium treatment is the greenish color which is imparts, detracting to some extent from the purity of the interference and absorption colors of the pigments. That disadvantage has now been overcome by stabilizing metal oxide-coated mica pigments with hydrous aluminum oxide.
Metal oxide-coated mica pigments have previously been treated with aluminum as part of other processes and for other purposes. In Bernhard U.S. Pat. No. 4,323,554, for example, aluminum hydroxide is precipitated onto metal oxide-coated mica in the presence of an organic compound and further treated with a dye to produce pigments coated with an aluminum lake.
Bernhard U.S. Pat. No. 4,490,179 teaches that a metal oxide-coated mica pigment can be made hydrophobic by coating with aluminum or chromic hydroxide followed by a carboxylic acid which bonds to the surface.
In spite of the number of investigations of the use of aluminum with metal oxide-coated mica, none has been devoted to deposition of a simple coating of hydrated aluminum oxide to effect stabilization against light and moisture.
It was therefore surprising to discover that such a coating increases resistance to ultraviolet light and moisture, making possible exterior grade mica pigments. Hydrous aluminum oxide has many advantages for this purpose: it is non-toxic, economical, and free of color.
The products of the invention are suitable for outdoor exposure and for applications in which light, moisture, or both may be encountered. They are especially useful for automotive finishes, paint on outdoor furniture, coil coatings, and incorporation in plastic signs, vinyl tiles, and the like.